Frequency of USP6 rearrangements in myositis ossificans, brown tumor, and cherubism: molecular cytogenetic evidence that a subset of "myositis ossificans-like lesions" are the early phases in the formation of soft-tissue aneurysmal bone cyst

Periosteal Fasciitis With Unusual Radiologic Features Harboring a Novel STAG1::USP6 Fusion Gene

Periosteal fasciitis is a subtype of nodular fasciitis originating from the periosteum. The diagnosis can be challenging and requires radiologic-pathologic correlation. Advances in molecular analysis confirmed that nodular fasciitis and its related lesions harbor a USP6 gene rearrangement with one of the several potential partners. Herein, we report a case of periosteal fasciitis with metaplastic bone formation detected incidentally during a radiologic survey for breast carcinoma. Radiologic examination revealed a 2.4 cm, heterogeneous, avidly enhancing lesion of the right femoral distal metaphysis concerning for low-grade periosteal chondrosarcoma. Histological examination of a core needle biopsy revealed a tumor composed of bland spindle cells with myofibroblastic and osteoblastic phenotypes admixed with immature bone and cartilaginous elements. Molecular analysis revealed a novel STAG1::USP6 fusion that helped arrive at the right diagnosis and further expands the molecular profile of USP6-associated neoplasms.

Unravelling the USP6 gene: an update

Ubiquitin-specific protease 6 (USP6) rearrangements have been identified in aneurysmal bone cyst, nodular fasciitis, myositis ossificans, fibro-osseous pseudotumour of digits and cellular fibroma of tendon sheath. These entities show clinical as well as histological overlap, suggesting they are all clonal neoplastic belonging to the same biological spectrum and referred to as 'USP6-associated neoplasms'. They all show a characteristic gene fusion formed by juxtaposition of the USP6 coding sequences to the promoter regions of several partner genes, leading to USP6 transcriptional upregulation.

Multimodality imaging features of USP6-associated neoplasms

Since the discovery of USP6 gene rearrangements in aneurysmal bone cysts nearly 20 years ago, we have come to recognize that there is a family of USP6-driven mesenchymal neoplasms with overlapping clinical, morphologic, and imaging features. This family of neoplasms now includes myositis ossificans, aneurysmal bone cyst, nodular fasciitis, fibroma of tendon sheath, fibro-osseous pseudotumor of digits, and their associated variants. While generally benign and in many cases self-limiting, these lesions may undergo rapid growth, and be confused with malignant bone and soft tissue lesions, both clinically and on imaging. The purpose of this article is to review the imaging characteristics of the spectrum of USP6-driven neoplasms, highlight key features that allow distinction from malignant bone or soft tissue lesions, and discuss the role of imaging and molecular analysis in diagnosis.

Case report: Large-size intramuscular nodular fasciitis, a challenging histopathologic diagnosis confirmed by molecular detection of USP6 gene rearrangement: Case report and literature review

The intramuscular subtype of nodular fasciitis (NF) is rare with lesions normally not more than 2 cm in size and characterized by pseudosarcomatous morphology. We report a case of a 27-year-old man with a large-size intramuscular NF. The patient came for treatment complaining of an increasingly enlarged mass in the left upper arm for 4 months. Magnetic resonance imaging (MRI) confirmed the presence of a well-defined tumor measuring 5 cm within the outer edge of the middle humerus. Microscopically, the neoplasm was rich in fibroblasts and myofibroblasts in an interlaced pattern with high mitotic index and evident multinuclear giant cells. Erythrocyte extravasation was easily seen in the stroma. The tumor border was infiltrative. Immunohistochemically, the tumor cells were positive for smooth muscle actin (SMA) and negative for cytokeratin, desmin, H-Caldesmon, CD34, S100, ALK, and β-catenin. Fibrosarcoma was highly suspected by histopathological and immunohistochemical examination. Molecular detection demonstrated evidence of ubiquitin-specific peptidase 6 (USP6) gene rearrangement in this tumor. Based on the findings, the tumor was diagnosed as intramuscular NF. At 56 months after the initial surgery, the patient had recovered with no evidence of recurrence or metastasis. Large-size intramuscular NF is very rare and easily overdiagnosed as malignant tumor due to its obvious pseudosarcomatoid pathological features. USP6 gene rearrangement detection can effectively avoid this major misdiagnosis.

USP6-associated soft tissue tumors with bone metaplasia: Clinicopathologic and genetic analysis and the identification of novel USP6 fusion partners

Introduction

Among those tumors with consistent USP6 rearrangement, some arise from soft tissue and show bone metaplasia, including myositis ossificans (MO), fibro-osseous pseudotumor of digits (FOPD), soft tissue aneurysmal bone cyst (ST-ABC) and fasciitis ossificans (FO). These lesions are easily confused with malignancies because they show a rapid growth rate and brisk mitoses. Here, we aim to clarify the clinicopathologic and genetic characteristics of this entity and analyze the correlations among the different subtypes in one of the largest cohorts.

Materials and Methods

The clinicopathologic features of 73 cases of MO, FOPD, ST-ABC and FO diagnosed at West China Hospital, Sichuan University from January 2010 to December 2021 were retrospectively analyzed. Forty-three undecalcified samples were analyzed by systematic genetic studies, including fluorescence in situ hybridization (FISH), reverse transcription polymerase chain reaction (RT-PCR), Sanger sequencing and next-generation-based sequencing were performed.

Results

This series included 40 males and 33 females aged 2 to 80 years old (median: 31 years). FOPD occurred in extremal soft tissue, while lower extremities (38/58, 65.5%) were the most commonly involved lesions in the other three subgroups. Histologically, proliferative myofibroblasts/fibroblasts with varying degrees of osteoid tissue were present. Fluorescence in situ hybridization (FISH) results indicated that 22 cases (22/27, 81.5%) were positive for USP6 rearrangement, and 5 cases were negative. Among those cases with positive FISH results, 18 underwent reverse transcription-polymerase chain reaction (RT-PCR) detection that successfully detected common USP6 fusion types. Thirteen cases showed COL1A1::USP6 fusion, one showed MYH9::USP6 fusion, and 4 were negative for common fusion types. Next-generation-based sequencing technology was performed on two lesions with negative RT-PCR results and novel fusion partners SNHG3 and UBE2G1 were discovered.

Conclusions

Our findings revealed that COL1A1 is the most common fusion partner in this entity, unlike primary aneurysmal bone cysts and nodular fasciitis. Notably, we believed that FO may demonstrate more similar clinicopathologic and genetic manifestations with MO/FOPD and ST-ABC instead of nodular fasciitis for involving lower limbs most frequently and showing recurrent COL1A1::USP6 fusion. Additionally, this study also found two novel USP6 fusion partners, which further expanded our knowledge of this neoplastic spectrum.

Case Report: Extraskeletal osteosarcoma with preceding myositis ossificans

Extraskeletal osteosarcoma (EO) is a soft tissue sarcoma characterized by the production of bone matrix by neoplastic cells. Benign osteoid in EO, leading to a diagnostic dilemma, is rarely encountered. Herein, for the first time, we present a case with cytogenetically confirmed EO combined with or preceding myositis ossificans (MO). A 21-year-old man had a mildly painful swelling in his left knee. Imaging studies demonstrated a 39-mm mass with peripheral mineralization and cystic change on the posterolateral side of the left fibular head. He was clinically suspected of having either MO or a malignancy, such that wide resection was performed. Macroscopically, the mass was grayish to brown. In the cut section, multiple cystic lesions in addition to solid components were noted. Histopathologically, the solid components demonstrated diffuse proliferation of pleomorphic tumor cells with osteoclast-like giant cells. The malignant tumor cells formed osteoid. In the periphery, the mass was benign, showing mature bone tissue and focally non-malignant woven bone with fibroblasts, compatible with zonation. Fluorescence in situ hybridization (FISH) demonstrated split signals of the USP6 gene. These findings suggested EO with preceding MO. Although the pathogenesis remains to be elucidated, the observed USP6 rearrangement might contribute to both the diagnosis of EO with preceding MO and an understanding of the underlying histopathology.

Cranial Fasciitis in Children: Expanding the Spectrum of USP6-Associated Clonal Transient Neoplasms

Background: Cranial fasciitis (CF) is a benign (myo)fibroblastic proliferation of children. Typical presentation consists of a rapidly growing solitary mass on the temporal or parietal cranium in the first 2 years of age. CF is characterized by a rapid growth followed by a relative slowdown and even growth arrest. The finding of somatic USP6 gene rearrangements demonstrating clonality in CF together with its clinical behavior places it in the category of diseases recently termed "transient neoplasia."Methods: Histological, immunohistochemical, and molecular findings of 18 patients with CF were retrospectively studied.Results: The tumor typically presented as a painless rapidly enlarging mass in the temporal region. Sixty-six percent of the cases harbored USP6 gene rearrangement. Nine patients were treated with gross total resection (GTR) and 9 with subtotal tumor resection (STR). Two patients treated with GTR had recurrence. Five patients treated with STR had progression-free disease for at least 10 months after surgery and in four patients the tumor regressed spontaneously a median 16 months after surgery.Conclusions: In this largest series to date, we reported the clinicopathological, immunohistochemical, and molecular findings of 18 pediatric cases of CF with emphasis on the clinical growth pattern of these tumors.

Update of pediatric soft tissue tumors with review of conventional MRI appearance-part 1: tumor-like lesions, adipocytic tumors, fibroblastic and myofibroblastic tumors, and perivascular tumors

There are numerous soft tissue tumors and tumor-like conditions in the pediatric population. Magnetic resonance imaging is the most useful modality for imaging these lesions. Although certain soft tissue lesions exhibit magnetic resonance features characteristic of a specific diagnosis, most lesions are indeterminate, and a biopsy is necessary for diagnosis. We provide a detailed update of soft tissue tumors and tumor-like conditions that occur in the pediatric population, emphasizing each lesion's conventional magnetic resonance imaging appearance, using the recently released 5th edition of the World Health Organization Classification of Soft Tissue and Bone Tumors as a guide. In part one of this review, pediatric tumor-like lesions, adipocytic tumors, fibroblastic and myofibroblastic tumors, and perivascular tumors are discussed. In part two, vascular lesions, fibrohistiocytic tumors, muscle tumors, peripheral nerve sheath tumors, tumors of uncertain differentiation, and undifferentiated small round cell sarcomas are reviewed. Per the convention of the WHO, these lesions involve the connective, subcutaneous, and other non-parenchymatous-organ soft tissues, as well as the peripheral and autonomic nervous system.

Ubiquitin-specific Peptidase 6 (USP6)-associated Fibroblastic/Myofibroblastic Tumors: Evolving Concepts

Ubiquitin-specific peptidase 6 (USP6) is a hominoid-specific gene residing on chromosome 17p13 and serves as a deubiquitinating enzyme with a diverse set of functions including intracellular trafficking, inflammatory signaling, cell transformation and protein turnover. USP6 rearrangements were first identified in aneurysmal bone cysts, resulting in promoter swapping and over-expression of wild type USP6. Several morphologically overlapping fibroblastic/myofibroblastic tumors are known to harbor USP6 rearrangements, including nodular fasciitis, cellular fibroma of tendon sheath, myositis ossificans and fibro-osseous pseudotumor of digits. Over the past few years, fusions involving the USP6 gene and various partner genes have been described in these neoplasms. The current World Health Organization Classification of Tumors of Soft Tissue suggests that USP6-rearranged lesions are typically benign and usually self-limited in their growth. This review provides an updated overview of the clinical, histological and molecular genetic features of USP6-associated fibroblastic/myofibroblastic tumors and discusses how these lesions should be best classified.

Clinicopathological and molecular characterisation of USP6-rearranged soft tissue neoplasms: the evidence of genetic relatedness indicates an expanding family with variable bone-forming capacity

AIMS

USP6 rearrangement underpins self-limiting fibroblastic/myofibroblastic neoplasms, including nodular fasciitis (NF), myositis ossificans (MO), aneurysmal bone cyst (ABC), and related variants. The aim of this study was to characterise UPS6 and fusion partners in order to delineate the clinicopathological, genetic and bone-forming features in such lesions of soft tissue (ST).

METHODS AND RESULTS

Break-apart fluorescence in-situ hybridisation (FISH) validated USP6 rearrangement in 31 of 35 NF [comprising three of three fasciitis ossificans (FO) cases, seven of eight cellular variant of fibroma of tendon sheath (C-FTS), four of six MO, three of three ST-ABC, and two of two fibro-osseous pseudotumours of digits (FOPD)]. As determined with FISH and reverse transcription polymerase chain reaction, MYH9-USP6 was the commonest fusion in four C-FTS and 20 NF, including one intravascular case and two infantile (one retroperitoneal) cases. The presence of MYH9-USP6 confirmed the diagnosis of two NFs> 50 mm with prominent ischaemic necrosis. COL1A1-USP6 was predominant in ossifying lesions, including all FO, MO, ST-ABC and FOPD with identified partner genes, and was also present in non-ossifying head and neck NF (HN-NF) and C-FTS in two cases each. A cervical NF of a 14-month-old girl harboured the novel COL1A2-USP6. Ossifying lesions showed considerable genetic and morphological overlaps. Sharing COL1A1-USP6, FO and FOPD showed similar central or haphazard bone matrix deposition. Besides zonation of outward bone maturation, four COL1A1-USP6-positive MO had incipient to sieve-like pseudocysts reminiscent of ST-ABC.

CONCLUSION

MYH9-USP6 is present in some C-FTS and most NF, including rare variants, but is unrelated to bone formation. All bone-forming USP6-rearranged lesions adopt COL1A1 as the 5' partner, indicating close genetic kinships. However, COL1A1/COL1A2 also contributes to the pathogenesis of minor subsets of non-ossifying USP6-rearranged HN-NF and C-FTS.

Knee intra-articular extraskeletal aneurysmal bone cyst: a case report and review of literature

Aneurysmal bone cyst (ABC) is a benign locally destructive lesion that usually developed in the bone cavity of bones, less commonly on the surface of cortical bones and very rarely develop outside the bone. There are only 35 reports of extraskeletal aneurysmal bone cyst (ESABC) in the English literature. We report a case of a 12-year-old female with no history of trauma who presented with knee pain. Imaging studies revealed an infrapatellar mass that was fast to calcify during a period of 3 months. MRI showed high T2 center, low T1 signal, and heterogenic enhancement with a rim of low intensity consistent with calcified boarders surrounded by severe soft tissue edema. The lesion was surgically excised, and a histological examination revealed an aneurysmal bone cyst possibly arising within myositis ossificans or heterotopic ossification. In her last follow-up 1.5 years after the surgical excision, the patient was symptom free and without signs of recurrence. To the best of our knowledge, this is the first reported case of an intra-articular ESABC located in the knee.

Benign infiltrative myofibroblastic neoplasms of childhood with USP6 gene rearrangement

AIMS

Several morphologically overlapping (myo)fibroblastic neoplasms harbour USP6 fusions, including aneurysmal bone cysts, nodular fasciitis, myositis ossificans, cranial fasciitis, fibro-osseous pseudotumour of the digits, and cellular fibroma of the tendon sheath. USP6-induced neoplasms are almost universally benign and cured by local excision. We aim to highlight the diagnostic value of USP6 fusion detection in a series of aggressive-appearing paediatric myofibroblastic tumours.

METHODS AND RESULTS

Three deep-seated, radiographically aggressive, and rapidly growing childhood myofibroblastic neoplasms were morphologically and molecularly characterised by USP6 break-apart fluorescence in-situ hybridisation (FISH), transcriptome sequencing, and targeted capture analysis. Each tumour occurred in the lower-extremity deep soft tissue of a child presenting with pain, limping, or a mass. In all three patients, imaging studies showed a solid mass that infiltrated into surrounding skeletal muscle or involved/eroded underlying bone. The biopsied tumours consisted of variably cellular myofibroblastic proliferations with variable mitotic activity that lacked overt malignant cytological features. FISH showed that all tumours had USP6 rearrangements. On the basis of these results, all three patients were treated with conservative excision with positive margins. The excised tumours had foci resembling nodular fasciitis, fibromatosis, and pseudosarcomatous proliferation. Next-generation sequencing revealed COL1A1-USP6 fusions in two tumours and a COL3A1-USP6 fusion in the third tumour. One tumour had a subclonal somatic APC in-frame deletion. No recurrence was observed during follow-up (8-40 months).

CONCLUSION

We present a series of benign, but aggressive-appearing, USP6-rearranged myofibroblastic tumours. These deep-seated tumours had concerning clinical and radiographic presentations and did not fit into one distinct histological category. These cases highlight the diagnostic value of USP6 fusion detection to identify benign nondescript tumours of this group, especially those with aggressive features, to avoid overtreatment.

Myositis ossificans-like soft tissue aneurysmal bone cyst: a clinical, radiological, and pathological study of seven cases with COL1A1-USP6 fusion and a novel ANGPTL2-USP6 fusion

Herein we described the clinical, radiological, histological, and molecular characteristics of seven soft tissue aneurysmal bone cysts (STABCs) diagnosed and managed at a tertiary cancer center and to elucidate their relationship with myositis ossificans (MO). All cases had established imaging and histopathological diagnosis of STABC and were subject to fluorescence in situ hybridization (FISH) for USP6 rearrangement and Archer® FusionPlex® targeted RNA sequencing (RNASeq) analysis to identify the fusion partner. A thorough literature review of STABC and MO was conducted. The patients presented with painful masses unpreceded by trauma, occurring most commonly in the deep soft tissue of the thigh/gluteus (4/7), and also in the supraclavicular region, the axilla, and the hand. On imaging, the lesions were frequently associated with peripheral calcification on conventional radiographs and CT (6/7), cystic components on ultrasound, as well as perilesional edema (7/7) and fluid levels (3/7) on MRI. Bone scan (1/1) showed intense radiotracer uptake. Histologically, 6/7 cases demonstrated zonal arrangements reminiscent of MO. USP6 rearrangement was found in all seven cases by FISH and/or RNASeq. RNASeq further detected COL1A1-USP6 fusion in six cases and a novel ANGPTL2-USP6 fusion in one case. Four patients underwent resection of the tumors and were disease free at their last follow-up. Three patients who underwent incisional or needle biopsies had no evidence of disease progression on imaging studies. In conclusion, the clinical, radiological, and pathological overlap between STABC and MO suggests that they are closely related entities. A novel fusion ANGPTL2-USP6 is associated with distinct clinical and pathological presentation.

USP6-Associated Neoplasms: A Rapidly Expanding Family of Lesions

Nearly 20 years ago, the first description of a translocation involving chromosome 17 on which USP6 resides was described. Since then, not only the culprit gene but also many fusion partners, leading to transcriptional activation of USP6, have been detected. The first neoplasm known to harbor USP6 rearrangements was aneurysmal bone cyst. Since then, other entities like nodular fasciitis, myositis ossificans, fibro-osseous pseudotumor of digits, and a subgroup of fibromas of tendon sheath, probably representing tenosynovial nodular fasciitis, have been added to the list of USP6-rearranged lesions. Remarkably, all of them share clinical as well as morphological characteristics, and authors have suggested that these entities actually belong to the same spectrum. This review summarizes the current knowledge regarding USP6-rearranged lesions and further elaborates on how these neoplasms relate to one another. We propose to call these lesions UAN (Usp6-associated neoplasm).

Pathologic evaluation of the solid variant of aneurysmal bone cysts with USP6 rearrangement with an emphasis on the frequent diagnostic pitfalls

The solid variant of aneurysmal bone cyst (SVABC) is very uncommon and frequently misdiagnosed. We reevaluated and summarized the clinicopathologic features of 17 SVABCs and further discussed the use of this nomenclature to differ SVABCs from extragnathic giant cell reparative granuloma (GCRG) in the setting of the USP6 rearrangement era. The immunohistochemical markers included α-SMA, SATB2, AE1/AE3, Ki67, S100, CD68 and P63. USP-6 status was detected by fluorescence in situ hybridization using a break-apart probe. The 17 patients with SVABCs comprised 10 males and 7 females ranging in age from 4 to 70 years. The involved locations included the long bone (n = 11), hand (n = 4), rib (n = 1) and vertebra (n = 1). The lesions were characterized by proliferated spindle cells with scattered giant cells and hemorrhages with variable positive α-SMA, SATB2, CD68 and Ki-67 expression. All patients had USP6 rearrangements without H3F3A glycine 34 mutations. Our study reveals that SVABC shares similar clinical and histologic features with other bone lesions, which may lead to an erroneous diagnosis. The presence of an USP-6 rearrangement contributes to the diagnosis SVABC; SVABC and most of the previously documented extragnathic GCRGs may be considered within the umbrella of primary aneurysmal bone cysts.

Rare aneurysmal bone cysts: multifocal, extraosseous, and surface variants

Multifocal, extraosseous, and surface aneurysmal bone cysts are rare variants of the primary lesions. The clinicopathological features are similar, and the optimal treatment is surgical. Although local recurrences may occur, the prognosis is excellent. This review article introduces the readers to a rare diagnosis which they may have been previously unfamiliar with, presents the clinicopathological and imaging features of these rare aneurysmal bone cyst variants, and discusses their diagnosis and treatment. The clinicians who treat patients with aneurysmal bone cysts should be familiar with these uncommon entities and their differential diagnosis.

Novel ASAP1-USP6, FAT1-USP6, SAR1A-USP6, and TNC-USP6 fusions in primary aneurysmal bone cyst

Aneurysmal bone cyst (ABC) is a benign but locally aggressive neoplasm, with a tendency for local recurrence. In contrast to other bone tumors with secondary cystic change, ABC is characterized by USP6 gene rearrangement. There is a growing list of known USP6 fusion partners, characterization of which has been enabled with the advent of next-generation sequencing (NGS). The list of known fusion partners includes CDH11, CNBP, COL1A1, CTNNB1, EIF1, FOSL2, OMD, PAFAH1B1, RUNX2, SEC31A, SPARC, STAT3, THRAP3, and USP9X. Using NGS, we analyzed a series of 11 consecutive ABCs and identified USP6 fusions in all cases, providing further evidence that USP6 fusions are universally present in primary ABCs. We identified four novel fusion partners in five ABCs and confirmed them by RT-PCR and Sanger sequencing, ASAP1, FAT1, SAR1A, and TNC (in two cases). Because of high sensitivity and specificity, detection of a USP6 fusion by NGS may assist in differentiating between ABC and its mimics, especially in small biopsy samples when a definite diagnosis cannot be achieved on morphological grounds alone. Further studies with a large number of cases and follow-up are needed to determine whether different fusion partners are associated with specific clinical and pathologic features of ABCs.

Cutaneous aneurysmal bone cyst-First report of a case and literature review

Aneurysmal bone cyst (ABC) is a benign, expansile, multiloculated and blood-filled cystic lesion most commonly involving bones. We report herein the case of a 78-year-old woman with a dermal ABC located in the skin of the right foot. Histologically, the lesion displayed characteristic features of ABC, including blood-filled cystic spaces lined by loose connective tissue, admixed with solid areas composed of spindle cells and osteoclast-like giant cells, associated with foci of woven bone formation and matrix calcification ("blue bone" formation). Fewer than 30 cases of extraosseous ABCs have been described in soft tissues and, to the best of our knowledge, ABC primarily occurring in the skin, not associated with an underlying lesion in the bone, has not so far been reported in the literature.

Soft tissue aneurysmal bone cyst: six new cases with imaging details, molecular pathology, and review of the literature

OBJECTIVE

Aneurysmal bone cysts (ABC) rarely present in soft tissue locations (STABC). The 30 cases of STABC reported in the English literature were reviewed. Six new cases retrieved from the files of the Netherlands Committee on Bone Tumors were compared to the six cases described in the radiological literature.

MATERIALS AND METHODS

Imaging studies and histopathology of six new STABC cases were reviewed. Follow-up was recorded with respect to local recurrence. FISH for USP6 rearrangement and/or anchored multiplex PCR-based targeted NGS using Archer FusionPlex Sarcoma Panel were attempted.

RESULTS

On imaging, the six STABC cases presented as a solid or multicystic intramuscular soft tissue mass, usually with thin peripheral mineralized bone shell. On MRI, perilesional edema was visualized in nearly all cases. Fluid-fluid levels were observed in one case. All lesions had the distinct histologic features of STABC. In three cases suitable for NGS, the diagnosis of STABC was confirmed by a COL1A1-USP6 fusion gene. In one additional case, USP6 gene rearrangement was detected by FISH. After marginal excision, none of the six STABC recurred after a mean follow-up period of 50 months (range, 39-187 months).

CONCLUSIONS

On imaging, it can be difficult to discriminate between STABC and myositis ossificans. The presence of a thin bony shell and fluid-fluid levels can be helpful in discriminating these two entities. STABC is readily diagnosed after histopathologic examination of the resection specimen. STABC belongs to the spectrum of tumors with USP6 rearrangements, which includes ABC, myositis ossificans, and nodular fasciitis.

Aneursymal Bone Cyst: An Uncommon Paraspinal Tumor in Children

Paraspinal tumors with benign histology in the absence of trauma rarely arise in children. Treatment of such benign tumors, in contrast to malignancies, generally consists of surgical resection of the lesion with confirmation of histology via pathologic evaluation. We present a pediatric case of an atraumatic paraspinal mass with a histologic diagnosis of aneurysmal bone cyst, and USP6 gene rearrangement supporting the histologic diagnosis. The patient underwent gross total resection of the paraspinal lesion with no additional intervention. We highlight the differential diagnosis of paraspinal tumors in children and key features that led to the diagnosis in this patient.

Solid aneurysmal bone cyst of the humerus mimics metastasis or brown tumor

Solid aneurysmal bone cyst (ABC) is a rare subtype of ABC that most commonly involves the small bones of the hands or feet. We present a case of a solid ABC of the distal humerus in a 52-year-old man with a history of chronic kidney disease and renal cell carcinoma. On imaging with plain radiographs, CT, and MRI, this expansile lucent lesion with solid internal enhancement had an appearance that overlapped with metastasis or brown tumor of hyperparathyroidism. On 18F-FDG PET-CT, this lesion was hypermetabolic with an SUVmax of 9.9. Only 37 cases of solid ABC have previously been reported to involve the long bones in the literature, and only 4 in the humerus. We review the clinical, imaging, and histopathological findings and differential diagnosis of solid ABC, and highlight the usefulness of identifying the USP6 gene rearrangement on FISH to distinguish this lesion from other lesions with secondary ABC formation.

Fluorescent In Situ Hybridization in Surgical Pathology Practice

There have been rapid and significant advances in diagnostic and predictive molecular techniques in recent years with profound impact on patient care. In situ hybridization (ISH) studies have become well entrenched in surgical pathology practice and their role in the evaluation of HER2 in breast carcinoma and their diagnostic utility in soft tissue pathology are well known. Fluorescent ISH is being increasingly used in other sites such as the head and neck and the gynecologic tract. Like most tests in surgical pathology, ISH studies require good quality tissue, correlation with clinical and histopathologic findings, and adherence to guidelines for optimal assay performance and interpretation. Although ISH studies are largely performed in tertiary centers, the tissue is often processed by a variety of laboratories and the referring pathologists are required to discuss the need, relevance, and significance of these tests and the results with their clinical colleagues. Here we review the predictive and diagnostic utility of fluorescent ISH studies in a variety of organ systems, the preanalytical factors that may affect the results, and the pitfalls in the interpretation that all practicing surgical pathologists should be aware of.

[Morphological spectrum of USP6 rearranged lesions]

USP6, also known as Tre-2 and TRE17, is an ubiquitase-specific proteinase that was identified more than two decades ago as a potential oncogene when it exhibited transforming properties upon overexpression in NIH 3T3 cells. Until recently, however, little was known about the function and the oncogenetic activation of USP6. The identification of rearrangements of the USP6 gene in aneurysmal bone cyst and in nodular fasciitis has not only led to a better understanding of the pathogenesis of these entities, but is also a useful tool in their diagnosis and differential diagnostic delineation from morphological mimics. In this review, the clinical, pathomorphological, and molecular genetic aspects of aneurysmal bone cyst and of nodular fasciitis, as well as from related lesions, are presented and discussed.

Extraskeletal aneurysmal bone cyst: Report of a case and review of the literature

Aneurysmal bone cyst (ABC) is an expansile cystic lesion that may affect any bone of the skeleton. Although exceedingly rare, lesions with histomorphologic characteristics of an ABC have reportedly originated within soft tissue. Extraskeletal ABC may mimic a variety of benign and malignant lesions and can be confused with other more common or rare giant cell-rich tumors of soft tissue, especially myositis ossificans. Clinical, radiological and histologic correlation is crucial in reaching the correct diagnosis. Cytogenetic and/or molecular genetic analysis is a useful adjunct in diagnosing these exquisitely rare lesions. Here we report a case of an ABC arising in an extraskeletal site and provide a comprehensive review of literature on this rare entity.

Clinical Applications of Molecular Markers in Bone Tumors

Pathologic diagnosis of primary bone neoplasms can be challenging primarily due to rarity of the disease entities, overlapping imaging and histologic findings, and lack of tumor-specific immunohistochemical stains. Although slow to evolve, in recent years there has been a rapid advance in the discovery of new and novel molecular markers in primary bone neoplasms, which has enhanced diagnostic accuracy and has shed light into their pathogenesis. Modern technological approaches such as next-generation sequencing including RNA sequencing are serving as "rapid discovery platforms" for new and novel mutations and translocations with diagnostic, prognostic, and possible therapeutic applicability. As next-generation sequencing technologies are finding their place in clinical laboratories, one could envision routine testing for mutations spanning across a gene or translocations with multiple breakpoints and partner genes. This review will focus on the clinical relevance and applicability of molecular markers in primary bone neoplasms with examples.

Soft-tissue aneurysmal bone cyst with translocation t(17;17)(p13;q21) corresponding to COL1A1 and USP6 loci

We present the case of a 46-year-old woman with no significant past medical history who developed left mid-thigh pain and fullness. Imaging demonstrated a mineralized soft-tissue mass, which increased in size during a year of monitoring, but retained a circumscribed appearance. The mass was located in the medial soft tissues of the thigh, separate from the bone on imaging studies, and this finding was confirmed during excision. The mass showed gross and microscopic features of an aneurysmal bone cyst. This diagnosis was supported by cytogenetic analysis revealing a t(17;17)(p13;q21) translocation corresponding to the USP6 and COL1A1 loci. Soft-tissue aneurysmal bone cyst is a rare entity, with fewer than 25 reports in the literature. Limited cytogenetic information about these tumors is available. To our knowledge, the USP6 and COL1A1 rearrangement has only previously been described in a pediatric soft-tissue aneurysmal bone cyst. We also discuss the differential diagnosis of ossifying soft-tissue lesions.

Chondro-Osseous Lesions of Soft Tissue

Soft tissue lesions can contain bone or cartilage matrix as an incidental, often metaplastic, phenomenon or as a diagnostic feature. The latter category includes a diverse group ranging from self-limited proliferations to benign neoplasms to aggressive malignancies. Correlating imaging findings with pathology is mandatory to confirm that a tumor producing bone or cartilage, in fact, originates from soft tissue rather than from the skeleton. The distinction can have dramatic diagnostic and therapeutic implications. This content focuses on the gross, histologic, radiographic, and clinical features of bone or cartilage-producing soft tissue lesions. Recent discoveries regarding tumor-specific genetics are discussed.

USP6 gene rearrangements occur preferentially in giant cell reparative granulomas of the hands and feet but not in gnathic location

Giant cell reparative granulomas (GCRGs) are lytic lesions that occur predominantly in the gnathic bones and occasionally in the small bones of the hands and feet. They are morphologically indistinguishable from, and are regarded as synonymous with, solid variant of aneurysmal bone cysts (ABC) in extragnathic sites. Identification of USP6 gene rearrangements in primary ABC has made possible investigating potential pathogenetic relationships with other morphologic mimics. USP6 gene alterations in giant cell-rich lesions (GCRG/ABC) of small bones of the hands and feet have not been previously studied. We investigated USP6 gene alterations in a group of 9 giant cell-rich lesions of the hands and feet and compared the findings with morphologically similar lesions including 8 gnathic GCRGs, 22 primary ABCs, 8 giant cell tumors of bone, and 2 brown tumors of hyperparathyroidism. Overall, there were 49 samples from 48 patients including 26 females and 22 males. Of the 9 lesions of the hands and feet, 8 (89%) showed USP6 gene rearrangements, whereas no abnormalities were identified in the 8 gnathic GCRGs, 2 brown tumors, or 8 giant cell tumors of bone. Of the 22 primary ABCs, 13 (59%) showed USP6 gene rearrangements. In conclusion, most GCRGs of the hands and feet represent true ABCs and should be classified as such. The terminology of GCRG should be limited to lesions from gnathic location. Fluorescence in situ hybridization for USP6 break-apart is a useful ancillary tool in the diagnosis of primary ABCs and distinguishing them from GCRGs and other morphologically similar lesions.

Cytogenetic and single nucleotide polymorphism array findings in soft tissue tumors in infants

Soft tissue tumors in children under one year of age (infants) are rare. The etiology is usually unknown, with external factors or congenital birth defects and hereditary syndromes being recognized in only a small proportion of the cases. We ascertained the cytogenetic findings in 16 infants from whom tumor tissue had been obtained during a 25-year period. In eight of them, single nucleotide polymorphism (SNP) array analyses could also be performed. No constitutional chromosome aberrations were detected, and assessment of clinical files did not reveal any congenital or later anatomical defects. Three tumors--one infantile fibrosarcoma, one embryonal rhabdomyosarcoma, and one angiomatoid fibrous histiocytoma (AFH)--had abnormal karyotypes. As the AFH had an exchange between chromosome arms 12p and 15q, additional fluorescence in situ hybridization and reverse transcription-polymerase chain reaction analyses were performed, unexpectedly revealing an ETV6/NTRK3 fusion. Three of the eight tumors, including the AFH with an abnormal karyotype, analyzed by SNP array showed aberrations (loss of heterozygosity or imbalances). The present series suggests that the addition of array-based technologies is valuable for detecting underlying pathogenetic mechanisms.

USP6-induced neoplasms: the biologic spectrum of aneurysmal bone cyst and nodular fasciitis

USP6 (also known as TRE17) is a ubiquitin-specific protease that was identified as an oncogene in transfection experiments with Ewing sarcoma DNA 2 decades ago. Until recently, little was known about USP6 function and mechanisms of oncogenic activation. The identification of USP6 fusion genes in aneurysmal bone cyst (ABC) and, more recently, in nodular fasciitis led to a better understanding of the pathogenesis of these lesions. Furthermore, the detection of USP6 genomic rearrangements or USP6 fusion genes may be used as a diagnostic tool for these lesions. In this review, we discuss the clinicopathologic features, molecular pathology, and pathogenesis of ABC and nodular fasciitis. We also discuss the possible line of differentiation of ABC and its relationship to nodular fasciitis and other lesions.

Molecular diagnostics complementing morphology in superficial mesenchymal tumors

Molecular techniques are increasingly important in the practice of surgical pathology. In soft tissue tumors, there are a number of tumors with recurring cytogenetic abnormalities. Knowledge of these abnormalities has furthered our understanding of these tumors and has also allowed development of molecular techniques to aid in the diagnosis. This review will focus on mesenchymal tumors with specific cytogenetic abnormalities that may present as a superficial tumor of the dermis or subcutis.

Updates on the cytogenetics and molecular cytogenetics of benign and intermediate soft tissue tumors

SOFT TISSUE TUMORS ARE CLASSIFIED ACCORDING TO THEIR HISTOLOGICAL RESEMBLANCE TO NORMAL ADULT TISSUES AND CAN BE GROUPED INTO THE FOLLOWING CATEGORIES BASED ON METASTATIC POTENTIAL: benign, intermediate (locally aggressive), intermediate (rarely metastasizing) and malignant. Over the past two decades, considerable progress has been made in our understanding of the genetic background of soft tissue tumors. Traditional laboratory techniques, such as cytogenetic analysis and fluorescence in situ hybridization (FISH), can be used for diagnostic purposes in soft tissue pathology practice. Moreover, cytogenetic and molecular studies are often necessary for prognostics and follow-up of soft tissue sarcoma patients. This review provides updated information on the applicability of laboratory genetic testing in the diagnosis of benign and intermediate soft tissue tumors. These tumors include nodular fasciitis, chondroid lipoma, collagenous fibroma (desmoplastic fibroblastoma), giant cell tumor of tendon sheath (GCTTS)/pigmented villonodular synovitis (PVNS), angiofibroma of soft tissue, myxoinflammatory fibroblastic sarcoma (MIFS) and ossifying fibromyxoid tumor (OFMT).

Molecular pathology and its diagnostic use in bone tumors

Bone tumors are considered by most pathologists difficult to diagnose as they are rare, have overlapping morphology, need radiological correlation, and the usefulness of immunohistochemistry is limited, making conventional morphology the cornerstone of the diagnosis. Over the past decade, more and more has become known of the molecular background of bone tumors. Three groups of bone tumors are recognized, namely, tumors with specific translocations combined with a relatively simple karyotype involving chromosomal translocations (Ewing sarcoma, aneurysmal bone cyst), tumors with specific gene mutations or amplifications (chondrosarcoma, fibrous dysplasia, chordoma), and sarcomas with genetic instability and as a consequence complex karyotypes (osteosarcoma). Technical advancements will rapidly reveal new alterations in the more rare sarcoma subtypes for which the molecular background has remained enigmatic. Opening the archives and using new technologies, as well as refinement of existing technologies for decalcified paraffin-embedded tissue, may bring to light more specific genetic aberrations in bone tumors that can be applied in molecular diagnostics in the near future.

Reactive bone lesions mimicking neoplasms

Reactive lesions of bone and soft tissue can appear alarming on histologic examination because they are often cellular and have atypical (activated) cytologic features, such as distinct nucleoli and mild hyperchromasia, and mitotic activity. Reactive lesions of bone and periosteum also produce bone and cartilage matrix, resulting in confusion with osteosarcoma or chondrosarcoma. Careful attention to key cytomorphological features such as the pattern of bone formation, uniform appearance of cells, and absence of atypical mitoses should help identify the reactive nature of a lesion. Correlation with clinical and radiological findings is also imperative to avoid misclassification of the tumor because reactive lesions often arise at sites where osteosarcoma and chondrosarcoma are rare (e.g., the hand) and lack aggressive radiological features. In this review we discuss reactive lesions of bone that are commonly confused with malignant neoplasms and that the practicing pathologist is likely to encounter at some point. Several of these lesions have had characteristic chromosomal translocations documented in recent years, but continue to be included as reactive lesions based on their overall clinicopathological features.

Pseudosarcomatous Soft Tissue Lesions: A Review

Pseudosarcomatous soft tissue lesions are potential pitfalls to both clinicians and pathologists due to their clinical presentation, resemblances to malignant lesions and their rarity. They also have a wide range of differential diagnosis histologically due to their relatively non-specific morphology and natural history of evolving features. The difficulty can be exacerbated by samples obtained from curettings or small biopsies as the architecture of the lesion is very helpful in pointing to the correct diagnosis. Familiarity with these entities is needed to avoid misdiagnosis and unnecessarily radical surgery. Despite recent advances in immunohistochemistry and genetic studies, their diagnosis still rest primarily on a combination of clinical history, radiological findings and histologic morphology.

TRE17/USP6 oncogene translocated in aneurysmal bone cyst induces matrix metalloproteinase production via activation of NF-kappaB

Aneurysmal bone cyst (ABC) is an aggressive, pediatric bone tumor characterized by extensive destruction of the surrounding bone. Though first described over 60 years ago, its molecular etiology remains poorly understood. Recent work revealed that ABCs harbor translocation of TRE17/USP6, leading to its transcriptional upregulation. TRE17 encodes a ubiquitin-specific protease (USP), and a TBC domain that mediates binding to the Arf6 GTPase. However, the mechanisms by which TRE17 overexpression contributes to tumor pathogenesis, and the role of its USP and TBC domains are unknown. ABCs are characterized by osteolysis, inflammatory recruitment, and extensive vascularization, processes in which matrix proteases play a prominent role. This led us to explore whether TRE17 regulates the production of matrix metalloproteinases (MMPs). In the current study, we demonstrate that TRE17 is sufficient to induce expression of MMP-9 and MMP-10, in a manner requiring its USP activity, but not its ability to bind Arf6. TRE17 induces transcription of MMP-9 through activation of NFκB, mediated in part by the GTPase RhoA and its effector kinase, ROCK. Furthermore, xenograft studies demonstrate that TRE17 induces formation of tumors that reproduce multiple features of ABC, including a high degree of vascularization, with an essential role for the USP domain. In sum, these studies reveal that TRE17 is sufficient to initiate tumorigenesis, identify MMPs as novel TRE17 effectors that likely contribute to ABC pathogenesis, and define the underlying signaling mechanism of their induction.